Conductor insulated with cellulose material



Sept. 7, 1937. T. R. scoTT ET AL 2,092,477

CONDUCTOR INSULATED WITH CELLULOSE MATERIAL Filed Jan. 15, 1932 TRSCOTT INVENTORS- AT TZRNEV Patented 7, 1937 UNITED STATES PATENT OFFICE 2,092,477 CONDUCTOR INSULATED WITH CELIQUI DSE MQTERIAL Thomas R. Scott and Malcolm 0. Field, London,

England, asslgnors to Western Electric Company, Incorporated, New York, N. Y., a corporation of New York Application January 15, 1932, Serial No. 586,914 In Great Britain January 2}, 1931 quantity of moisture and ionizable material associated with the cellulose micelles or fibres under operating conditions of humidity. When the latter is high, the cellulose insulations at present in use are unsatisfactory, and in order to overcome these undesirable properties several attempts have been made to treat the cellulosic material by some such process as careful wash-' ing. Although these processes are known to reduce the quantity of ionizable constituents present with a corresponding slight decrease in water absorptive properties and improvement in insulation value, the gain is not suflicient for all types of insulation and further improvement is desirable.

An object of this invention is to produce a conductor electrically insulated with a material exhibiting the fibrous structure and consequent mechanical strength typical of cellulosic bodies which have reduced moisture absorption and improved insulation value.

According to one feature of the present invention, a conductor is insulated with a cellulosic ilbrous'material in the form of yarn, woven fabric, paper, twine or disintegrated fibres subsequently formed into yarn, fabric, paper, twine, etc., which ha been subiectedto a reagent or mixture of reagents which gradually convert the cellulose into esters or ethers, the process being conducted for a period of time depending on the particular reagent or mixture reagents employed and the temperature, and determined by the electrical characteristics of they product after washing and drying, in such a way that the insulation resistance of the product is greater than the insulation resistance of the untreated material or the untreated material which has merely been subjected to a careful washing process without any chemical change in the structure of the cellulose when tested under identical conditions of humidity, temperature, etc., the reagent-or mixture of reagents having this characteristic, that during the time of treatment as above determined, they do not markedly alter the mechanical or physical structure of the fibres or do not markedly reduce their mechanical strength. Thus, the process may in practice be determined as regards reagents and time on a test sample giving an insulation resistance of a length of the material greater than megohms per gm. at 70% relative humidity and temperature of 40 C. Alternatively, the process maybe determined as regards said factors as giving a test sample whose moisture absorption is less than 4% by weight in the'case of textiles and 6% by weight in the case of papers at 70% relative humidity and a temperature of 20 C.,

the dry weight basis being taken after desiccating for 48 hours over phosphorous pentoxide in vacuum.

The nature of these and other features of the invention .will be more readily understood from the following description of examples of carrying out the invention and the accompanying drawing, the single figure of which illustrates one embodiment of the invention wherein an individual conductor is insulated with a layer of paper which has been partially esterifled or etherifled. In accordance with this drawing the conductor 5 is provided with a layer of paper or hanks is immersed in a well stirred bath comprising acetic anhydride, a dehydrating catalyst and glacial acetic acid as a diluent for a time suflicient to convert the cellulose present into'a partially acetylated derivative showing on analysis an acetic acid content equivalent to approxi mately the mono-acetate (29.4% by weight) after which the esterified productis carefully washed with wateruntil all traces of the reagents have disappeared.

An acetylating bath in which the proportions are acetic anhydride, 10% anhydrous zinc chloride and 70% glacial acetic acid by weight) has been found suitable and takes about 20 hours to convert the fibres to the mono-acetate at 40 C.

A'sample oi the product was found to have a moisture absorption of less than 4% when dried out for, 48 hours in vacuum over, phosphorous scribed above. It is, however, not advisable to use too high a temperature, as this results'in uneven acetylation, increased loss of material due to the formation of cellulose tri-acetate which is soluble in the bath and destruction of the form and structure of the yarn.

Certain other partial cellulose esters and ethers yield suitable products such as methyl, ethy1 and 0 phenyl cellulose. In each case the cellulose is reacted upon by an oxygenated organic com-' pound in such a way as to form the corresponding derivative and the reaction is not allowed to proceed to the stage where the mechanical structure of the cellulose fibers is materially altered. In the formation of the ester, the cellulose fibers may be reacted upon, for example, by an acid anhydride .or an acid chloride. In the formation of the ether the cellulose fibers may be reacted upon by an acyl or alkyl halide in such a way as to form the corresponding ether.

In general, the known method of preparing the fully esterified or etherified product is adopted and moderated by reducing the concentration of 3 the active reagents or by reducing the temperature of the reaction (in a. similar manner to that described above for the acetylation) whereby the cellulosic material-can be removed at the desired stage equivalent to the formation ofa partial ester or ether with the required characteristics vof mechanical strength, fibrous structure, moisture absorption and insulation value cited already.

' It should be understood that in the acetyla- 46 tion of'cellulose, this invention is not limited to the above typical bath compostion but that the proportions of the constituents can be varied between quite wide limits and still yield satisfactory results, provided the other variables, 1. e.,

50 time of acetylation, temperature of acetylation,

type of catalyst, etc, are carefully chosen For most purposes the acetylation is stopped approximately at the mono-acetate stage v(i. e. an acetic acid content by analysis of.29.4% by weight). From experimental curves it is pos-- sible to gauge the time necessary to complete the reaction to this stage. The mono-acetate is preferred because this shows the optimum conditions of mechanical strength and moisture absorption reduction and electrical insulation value.-

The fact that the monoand di-acetates of cellulose cannot be separated in the pure state (the terms mono- -di-acetate referring in this specification to the mean analytical value) it is apparentthat all degrees of acetylation from zero to that equivalent to the tri-acetate can be obtained. For certain purposes it may be desirable to increase, the acetic acid content to a value beyond that equivalent to the mono-acetate, i. e. up to 48.8% which is equivalent to the di-acetate, this is accomplished by a. longer period of acetylation.

In order to conserve the acetic anhydride repentoxide and then placed in an atmosphere of agent, it is usual in practice to have present only a. slight excess beyond the theoretical quantity for the particular acetate required, thus theoretically 62.9 grams are required to transform 100 grams of cellulose in 125.9 grams of cellulose larger mono-acetate and a proportionately quantity for the di-acetate. Usually -100 grams would be employed in the former instance to allow for that decomposed by the water present in the fibres and that lost by the formation of small quantities of cellulose tri-acetate.

It is obvious that by limiting the quantity of acetic anhydride that the reaction may be stopped at any desired stage.

In order to obtain uniform acetylation, it is sometimes desirable to pre-treat the cellulose fibres to remove all traces of fatty or waxy matter and nitrogenous compounds. In the case of cotton yarn this is readily achieved by Kier boiling, i. e. boiling with 2% caustic soda solution in theabsence of air under three atmospheres pressure, followed by thorough washing. Other well known methods may be used and prove equally satisfactory.

Dehydrating catalysts hitherto employed for the preparation of cellulose tri-acetate may be used to accelerate the esterification provided that the quantity of the more strongly acting types is reduced to permit even acetylation of the fibres.

Homogeneous partial acetylation of cellulose fibres may be carried out equally well in two stages, so that the cellulose material is first treated with one or two of the constituents of the acetylating mixture, the remaining constituent or constituents being subsequently added.

In practice the theoretical yield of cellulose monoor di-acetate is never realized owing to a slight loss occurring due to the formation of traces of cellulose tri-acetate which are soluble in the bath, this only becomes pronounced if acetylation is continued beyond the mono-acetate stage. It is advantageous in that it ensures that no tri-acetate is present in the product.

The acetylating bath may be used repeatedly provided deficiencies of acetic anhydride and dehydrating catalyst are made good.

The following figures for the moisture absorption of partially acetylated cotton were obtained on samples of 2/30s washed and purified cotton acetylated in the bath of composition already described at 20 C.

'For comparison figures for several other commercial cottons and acetate'silks are included, together with samples of partially acetylated cottons acetylated in baths of different compositions.

Percent Percent moisture absorption 355 35 combined at a relative humidity of Sample 7 acetic tion in acid 0 1mm content 25. 5% 50% 70.8% 88.8% 100% 2/308 washed 14% 0 3. 24 5. 27 7. 42 11. 11 15. 24 and purified 22% 5 2. 81 5. 01 7. 40 10. 26 i3. 66 cotton par- 38% 10. 0 2. 29 4. 41 6. 39 8. 61 13. 05 tially 8081ZY- 62 18. 4 2. 06 3. 57 5. 30 7. 83 10. 86 18M- 86 25. 8 1. 46 2. 4. 65 6. 9. 61 134% 38. 3 l. 06 2. 28 3. 92 6. 11 8. 77 182 45. 6 1. 05 2. 28 3. 85 6. 28 8. 64 Partially acet- 29. 14 1. 05 2. 21 3. 1s 5. s7 7. 99 2 ylated 2/308 29. 58 0. 95 2. 09 3. 51 5. 53 6. 97 cotton.

Partially acetylated 2/268 2B. 0 1. 08 2. 20 3. 73 6. 00 8. 23 cotton.

Time 01 Percent Percent moisture absorption 1 combined at a relative humidity oi Sample may acetic tionin acid 5 1 conte 21159:, 50% 10. 0% seat; 100% Partiall acetyla cot- 29.51 0.86 2.05 355 5.91 8.18 ton tape. Commercial 51.75 1.53 3.63 5.69 8.45 11.55 celluloseaw 1.81 4.05 6.98 10.49 14.58 tatc 1.49 3.30 5.51 8.48 11.49 1.54 3.49 5.20 9.55 14.42 am; cotton 2.00 5.19 7.5% 11.33 15.20 yarn sam- 2.85 5.19 7.25 9.64 1209 pies.

. 2130s washed 3.01 5.00 133 011 14.30 cotton yarn 3.04 5.32 751 981 13.05 samples.

2/30s washed and purified 2.90 4.92 7.06 10.74 14.86 cotton yarn.

The following figures for the insulation resistance and mechanical strength of 2/30s cotton treated after the manner described therein turther illustrate the invention:-

drawing or for the dielectric for condensers and for similar Purposes.

In the paper esterified in the above described manner the strength and structural desiderata will be retained but the amnity for moisture due to exposed cellulose surface will be reduced considerably. Thus, in either unimpregnated or impregnated conditions the insulating qualities of the paper will be considerably improved.

Moreover, owing to the changes in fibre bulk during esterlfication and owing to the reduced specific inductive capacity of cellulose esters compared with pure cellulose, it should be possible to control the impregnated and unimpregnated characteristics (dielectric) of'the paper during esterification to a degree that cannot be attained during. the manufacture of the paper itself.

Whatis claimed is:

1. An electric conductor comprising a conducting core and a layer oi paper surrounding said core, said paper consisting oi cellulose fibers which have been converted into partial ethers without materially aflecting their mechanical structure.

Insulation resistance per gram of textile measured on a length thread with anapplied voltage of 96 volts D. C.

80 Tem- Relative humidity rg' Textile 25C. Partially acetylated cot- X X ton (2/30s)... 150,000,000 45,000,000 12,000,000 2,800,000 500,000 70,000

(29.58% combined ace tic acid) Cellulose acetote s1 2,(XX),000 1,4(X),000 8M0!) 5(1), 22ml!) 50,000

Tem- Relative humidity m' Textile i C. v 40% 45 Untreated cotton; 3,2) 8t!) 28 5. 0 O. 7

40 0. Partial] acet- YlB .cot- X X X 1 ton (2/33)"- 400,000,01!) 56,(X)0,(ID ZSJIXLMO 7,000,0(D1,580,(Xl) 3100!) (29.58% com- I 50 bined acetic acid) Cellulose acetate si1k SGMXX) 560,0) 380,0m 250,(XX) IZLM 50,

U n t r e a t e d I 55 -ootton e00 225 63 16 3.9 0.7

X. These figures were obtained by exterpolation as the actual measurements were beyond the sensitivity of the measuring instruments.

Tensile strength measured on 18" length at a Conductors insulatedwith material herein described are particularly suitable for use in humid atmospheres. Where paper is treated in accordance with the above methods it may be used for cable insulation in the'manner disclosed in the Y 2. An electric conductor comprising a conducting core and a layer of paper surrounding said core, said paper consisting of partially etherified cellulose fibers having substantially the same mechanical structure as the original cellulose fibers and selected from the group which consists of methyl cellulose, ethyl cellulose and phenyl cellulose.

3. An electric conductor comprising a conducting core and a layer of paper surrounding. said core, said paper consisting of partially etherified methyl cellulose in which the mechanical structureof the original cellulose fibers is substantially retained. v

4. An electric conductor comprisinga conducting core'and a layer of paper surrounding said. core, said paper consisting of partially said core, said paper consisting of partially etherified ethyl cellulose in which the mechanical etherifled phenyl cellulose in which the mechanistructure of the original cellulose fibers is subcal structure of the original cellulose fibers is stantiaily retained. substantially retained.

5. An electrical conductor comprising a con- THOMAS R. SCOTT.

' 5 ducting core and a layer of paper surrounding MALCOLM C. FIELD. 

